Zeolites are a microporous crystalline material formed by a matrix of TO4 tetrahedrons that share all their vertices giving rise to a three-dimensional structure that contains channels and/or cavities of molecular dimensions. They are of variable composition, and T generally represents atoms with formal oxidation state +3 or +4, such as for example Si, Ge, Ti, Al, B, or Ga. When some of the T atoms have an oxidation state less than +4, the crystalline matrix formed presents negative charges that are compensated by means of the presence in the channels or cavities of organic or inorganic cations. These channels and cavities may also contain organic molecules and H2O, therefore, in a general manner, the chemical composition of the zeolites may be represented by means of the following empirical formula:x(M1/nXO2):yYO2:zR:wH2O
where M is one or several organic or inorganic cations of charge +n; X is one or several trivalent elements; Y is one or several tetravalent elements, generally Si; and R is one or several organic substances. Although by means of postsynthesis treatments the nature of M, X, Y and R and the values of x, y, z, and w may vary, the chemical composition of a zeolite (just as is synthesized or after its calcining) possesses a characteristic range for each zeolite and its method of preparation.
The crystalline structure of each zeolite, with a system of channels and specific cavities, gives rise to a characteristic diffraction pattern of X-rays, which allows one to differentiate them from each other.
Many zeolites have been synthesized in presence of an organic molecule that acts as a structure director agent. The organic molecules that act as structure director agents (SDA) generally contain nitrogen in their composition, and they can give rise to stable organic cations in the reaction medium.
The mobilization of the precursor species during the zeolites synthesis may be carried out in the presence of hydroxyl groups and basic medium that can be introduced as hydroxide of the same SDA, such as for example tetrapropylammonium hydroxide in the case of the zeolite ZSM-5. The fluoride ions can also act as mobilizing agents in synthesis of zeolites, for example in the patent EP-TO-337479 the use of HF is described in H2O at low pH as a mobilizing agent of silica for the zeolite ZSM-5 synthesis.